Common methods to measure an electrical resistance of an electrically conducting element are (1) applying or forcing a known constant voltage across the two terminals comprising the resistive element and measuring the resulting electric current; and (2) forcing a known, constant current through the resistive element and measuring a resulting voltage drop across the element's terminals. The resistance is thus defined as the ratio of voltage to current under static (non-time-varying) conditions.
Any additional unknown resistance (e.g., wiring or switches) in series with the resistive element contributes to the measured resistance, creating a source of error. If the resistance of interest is not much larger than such additional parasitic resistance, then an alternative method is employed which relies on two additional terminals which perform a sensing function (often referred to as a Kelvin method). FIG. 1A illustrates a prior art approach for sensing the resistor terminal voltage when forcing a voltage across the resistive element. FIG. 1B illustrates another prior art approach for sensing the resistor terminal voltage when forcing a current across the resistive element.
In FIG. 1A, the applied voltage difference V1−V2 induces a current in the test resistor 101 (R) which produces the voltage difference V1′−V2′. The voltage difference is sensed by forcing a negligible current through each sense terminal (i.e., sense terminals 1 and 2) and measuring a resulting voltage on the sense terminals. The current, I, flowing through either terminal 1 or terminal 2 is measured, and the test resistance determined as R=(V1′−V2′)/I. Since negligible current is passing through parasitic resistances rsense1 and rsense2, they do not contribute significant error to a determination of the current I. Nor do rsense1 and rsense2 introduce a significant voltage drop, allowing V1′ and V2′ to be directly associated with voltages measured on their respective sense terminals. In addition, additional parasitic resistances r1 and r2 do not impact the accuracy of the measurement since V1′ and V2′ are sensed directly.
FIG. 1B illustrates a similar configuration differing from FIG. 1A only in that current flowing through a test resistor 151 (R) current is forced directly by a current source 151 (I=Ibias). In both configurations, the measurement apparatus must measure the voltages V1′ and V2′ in order to determine the resistance of the resistor 101.
In the event that the parasitic resistances r1 and r2 are very large compared with the test resistor 101, accuracy limitations arise. For a given force current (Ibias), or applied voltage V1−V2, the voltage drop across the test resistor 101 scales with a proportionality R/(R+r1+R2). If the voltage drop across the test resistor 101 becomes sufficiently small, noise-induced voltage fluctuations in the measurement system can cause accuracy degradation and/or long required test times. Test instrumentation can minimize random noise error but at the expense of using time averaging techniques, thereby increasing test time (often considerably). Resistance monitoring applications in a production manufacturing environment may not be able to accommodate a resulting reduction in throughput.
U.S. Pat. No. 6,013,952 to Chan and U.S. Pat. No. 6,362,638 Ashton, et al. each describe an application for measuring the interfacial resistances of integrated circuit films which are representative of the problem described above. However, the small interfacial resistances are measured using a force/sense method which must resolve small voltage drops. Long test times can result to counteract the adverse effects of system noise when measuring small resistances.
Therefore, what is needed is an electrical resistance measurement apparatus and method capable of both accurately measuring small values of resistance and in a time-frame conducive to production environments. The apparatus and method should further be capable of accurately measuring the small resistance value in the presence of series-connected parasitic resistance values that may be orders of magnitude larger than the resistor under test.